The present invention concerns electric machines, and especially drive motors for signal-processing equipment mainly adapted for slow-operation, such as tape recorders and the like.
Drive motors of the type in question are disclosed in U.S. Pat. 3,845,339 and in U.S. patent application Ser. No. 708,092, now U.S. Pat. No. 4,174,484. In addition to requirements concerning simple construction and axial compactness, when such motors are rpm-regulated there arises a further problem, involving the need to suppress the effect upon the motor's rpm-regulating circuit of external interference coming from outside the motor, e.g., coming from electronic components of the apparatus driven by the motor, coming from closely located transformers, and so forth. In order to generate the actual-rpm signal needed for negative-feedback regulation of rpm, use is conventionally made of a tachometer generator. The shielding required for the latter, in order to shield it from the interference emanating from neighboring components and from the drive motor itself, generally requires a considerable amount of expense.